(a) Field of the Invention
The present invention relates generally to coding schemes for communication systems and, more particularly, to a two-thirds rate short block coded modulation scheme for use in Rayleigh fading channels.
(b) Description of Related Art
Digital communication techniques and technology are used in many different applications including satellite and cellular communications systems. One critical aspect of digital communications is a receiver""s ability to distinguish one received signal from another. In a digital communication system, symbol separation is one way to prevent confusion between received signals.
Hamming distance and product distance are two well known metrics that are used to measure the separation of communication codewords in a digital communications system. In general, the amount of separation corresponds to the ease with which communication codewords will be confused (i.e., the smaller the separation between two symbols, the more likely it is that the two symbols will be confused). Hamming distance (also referred to as branch diversity) represents the number of symbols that are different between two codewords. For example, the Hamming distance between binary codewords 010 and 101 is three, and the Hamming distance between 010 and 000 is one. The greater the Hamming distance between two codewords, the less likely it is that the two codewords will be confused.
Another useful metric that is considered in digital communications systems is Euclidean distance, which is the measurement of the separation between transmitted codewords as they are modulated onto a carrier. Specifically, Euclidean distance is the measure of the distance between two symbols on the communication codeword bit-map, or constellation. For example, if a carrier phase of 0xc2x0 represents the codeword 001 and a carrier phase of 180xc2x0 represents codeword 011, the two codewords have maximum Euclidean distance. Conversely, if the codeword 001 is represented by a phase of 0xc2x0, and the codeword 100 is represented by a phase of 45xc2x0, the Euclidean distance is a minimum.
Two common communication channel models are the Gaussian channel and the Rayleigh channel. A Gaussian channel is a channel having an additive noise factor. In a Gaussian channel, the power received by a receiver is essentially the power transmitted by a transmitter, reduced by a distance attenuation factor, coupled with a noise power component. When attempting to send information through a Gaussian channel using a phase shift keying technique (e.g., PSK, QPSK, QQPSK) it is useful to maximize the Euclidean distance between codewords to minimize the error probabilities between the transmitted phases.
A Rayleigh communication channel is characterized as a fast fading channel having multipath communication components that interact constructively and destructively to distort the amplitude of the signal received at a receiver. The multipath components cause received power to vary between zero and twice the power transmitted, depending on the type of interference taking place. Because received power can drop out completely in a Rayleigh channel, Euclidean distance is not critical because the underlying principle behind Euclidean distance is the elimination of the confusion between the phases of the various received signals. In a Rayleigh channel, the signal may not even be present. Therefore, the Hamming distance is critical to system operation in a Rayleigh channel. If the Hamming distance between two signals is large, a partial loss of the received signal may not necessarily inhibit the determination of the transmitted codeword.
The Rayleigh channel model is commonly used when designing a mobile communication system such as cellular, wireless or local network system. One known coding scheme that functions in a Rayleigh channel is the trellis code modulation scheme. Trellis coding has three main features. First, trellis coding provides additional points in the communications constellation for redundancy and forward error correction. Second, trellis coding uses convolutional coding to introduce dependency between successive signal points, such that only certain signal patterns are permitted in the system. Third, soft decision coding is performed at the receiver in which the permissible sequence of signals is modeled as a trellis structure. Trellis coding has the disadvantage of being computationally intensive and, therefore, slow and occasionally difficult to implement in systems not having large computational capacity.
A method of constructing a communication code, including the steps of generating a first set of codewords, generating a second set of codewords, generating a third set of codewords by combining the first set of codewords with the second set of codewords, and generating a fourth set of codewords by combining the first set of codewords with the second set of codewords. The first set of codewords includes 128 hand-optimized codewords and the second set of codewords includes 256 hand-optimized codewords. According to the present invention, the third and fourth sets of codewords include 32768 codewords.
According to the present invention, the step of generating the third set of codewords includes multiplying each codeword in the first set of codewords by a multiplication factor to create a set of multiplied codewords and adding each of the multiplied codewords to each of the second set of codewords. Additionally, the step of generating said fourth set of codewords comprises multiplying each codeword in the second set of codewords by a multiplication factor to create a set of multiplied codewords and adding each of the multiplied codewords to each of the first set of codewords.